1. Field of the Invention
The present invention relates to a power transmission device and, more particularly, is preferably used in a compressor for a vehicle air conditioner operated by an external power source, such as an engine, via a belt etc. by being incorporated therein.
2. Description of the Related Art
A refrigerant compressor for vehicle air conditioner is driven by an external power source, such as an engine, via a belt, pulley, etc., and an electromagnetic clutch may be inserted therebetween in order to separate connection between the engine and the compressor. If, however, an electromagnetic clutch is not inserted, the cost is reduced, and therefore, in some cases, an electromagnetic clutch may be omitted. In this case, in a power transmission device of a compressor for a vehicle air conditioner operated by an external power source such as an engine via a belt etc., a torque limiter is installed in order to avoid a trouble, such as belt damage, if the compressor seizes.
Torque limiters include one in which a part of the power transmission path is threadedly joined to utilize an excessive axial force generated at the threadedly joined part due to an excessive torque when the compressor seizes (for example, refer to patent document 1). As described above, in a conventional power transmission device for transmitting power to a compressor, a power transmission shut-off device (torque limiter) is arranged in order to avoid a trouble, such as damage to a belt for power transmission, if the compressor seizes. Conventionally, a power transmission shut-off device (torque limiter) having a structure in which a portion of a power transmission portion is threadedly inserted has been proposed and a torque limiter system that utilizes threaded insertion is a system for cutting off the power transmission path by rupturing a portion of the power transmission path using an excessive axial force generated at the threadedly inserted portion by the excessive torque generated when the compressor seizes. In other words, the torque limiter system has a structure in which the power transmission shut-off member is broken by a tensile force of the excessive axial force generated by threaded fastening using the excessive torque generated by the seizing phenomenon of the compressor. However, when the power transmission shut-off member ruptures, depending on the shape of the ruptured part, there is a possibility of a situation where the thread portion of the power transmission shut-off member is re-fastened. In this case, a gap space formed by the power transmission shut-off member and the bearing surface of an inner hub accommodating the power transmission shut-off member is eliminated and the bearing surface of the inner hub is sandwiched by the thread portion of the power transmission shut-off member and the shaft end surface of the compressor, and thus there has been a trouble that power cannot be shut off.
A conventional power transmission device 50 having the above-mentioned problem is explained with reference to FIG. 19 to FIG. 21. FIG. 19 shows a partial side section view of a conventional example of a power transmission device utilizing threaded fastening and FIG. 20 and FIG. 21 are partial side section views showing two states in which the torque limiter (power transmission shut-off member) of the power transmission device 50 shown in FIG. 19 has operated and ruptured. In the configuration of the power transmission device 50 shown in FIG. 19, rotation of a power source such as an engine is transmitted to a pulley via a belt etc. and, further, is transmitted to the power transmission device 50 assembled to the pulley. The configuration of the power transmission device 50 is fundamentally the same as that of the power transmission device of the present invention shown in FIG. 1 etc. and will be described later. Therefore, the details are not explained here. In the power transmission device 50, power is first transmitted to a hub and then transmitted in the order of the power transmission shut-off member and the rotating shaft of the compressor etc. threadedly fastened to the power transmission shut-off member.
The power transmission device 50 in the conventional example in FIG. 19 is assembled by threadedly coupling a thread part 303 of a power transmission shut-off member 3 to a thread part 402 of a rotating shaft 4 and moving it in the direction toward a device to be driven or the rotating shaft (in the rightward direction in FIG. 19). Due to this, an inner hub 204 of a hub 2 is sandwiched between, and compressed by, the power transmission shut-off member 3 and the rotating shaft 4. In this manner, by the axial compression force due to the threaded coupling of the power transmission shut-off member 3 and the rotating shaft 4, a frictional force is produced at a contact surface at which the power transmission shut-off member 3 and the inner hub 204 come into contact with each other and at a contact surface at which the inner hub 204 and the rotating shaft 4 come into contact with each other. Due to the frictional force, the power transmission shut-off member 3 and the hub 2 rotate, as one body, together with the rotating shaft 4.
FIG. 20 and FIG. 21 are diagrams for explaining a configuration after the power transmission shut-off member 50 has operated and the power transmission shut-off member 3 has ruptured. In FIG. 20, if the power transmission shut-off member 3 ruptures at a rupture part 301, ruptured surfaces 308 and 309 are formed and the power transmission shut-off member is split into a flange part 302 and a thread member 306. The ruptured surfaces 308 and 309 are not necessarily flat plane-shaped. Therefore, in FIG. 20, as the flange side ruptured surface 308 rotates the thread member side ruptured surface 309, the thread member 306 of the power transmission shut-off member advances toward the side of the compressor (device to be driven). Due to this phenomenon, the gap “B” 9 formed in FIG. 19 and FIG. 20 is eliminated, an end surface 307 of the thread member 306 of the power transmission shut-off member interferes with an opposite contact surface 204e of the hub, and further, a hub bearing 204c is sandwiched by the end surface 307 of the thread member 306 and a shaft contact surface 403 on the rotating shaft side, and thus power is transmitted. In other words, even if the power transmission shut-off member 3 has operated and ruptured, power is not shut off and a problem arises that the power is transmitted to the rotating shaft and further to the device to be driven such as the compressor.
[Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 2003-206950